In modern looms, the axial dimension of the heddle frames and of all the lever parts making up said main control levers has been standardized at 12 mm, and because of the considerable reciprocating loads that have to be transmitted the cheeks of the main control levers cannot have a thickness less than about 2.5 mm. Since a certain axial clearance has to be allowed, the axial thickness of said cam-operated crank arms cannot exceed about 6 mm, and consequently the two roller bearings of said crank arms must necessary also have that axial thickness. In the current state of the art, the large-diameter rolling-contact bearing of small axial dimension between the cam and the driving crank arm is formed from balls housed in races provided directly in the cam and crank arm and kept spaced apart by a suitable spacing cage.
Such an arrangement gives rise to a series of drawbacks in its operation, control, construction and assembly.
In this respect, considerable difficulty arises in machining the ball races, which have to be formed with a very close tolerance of some thousandths of mm in large-dimension but small-thickness workpieces and are therefore easily misshaped. These difficulties are aggravated by the fact that the grooves have then to be thermally hardened. The evident problems of controlling defects in said grooves are further supplemented by the laborious operations involved in mounting the balls equidistantly and installing the spacer cage. Finally, but not lastly, it is not possible to completely fill them with balls, as only that number of balls can be mounted which can be inserted between the cam and crank arm when the cam is held with one side in contact with said crank arm, thus limiting the load-carrying capacity of the bearing.
This latter drawback is the most serious considering the increasingly higher speeds used in modern looms. Since the loads to be supported increase with the cube of the speed, this requires the construction of bearings which for a given diameter and a constant fixed axial dimension are able to support a substantially increased load.